In short:Because the concept of a “self” – itself! – prescribe
that this “ego” remains exactly the same
over a prolonged time!
So also do we subjectively indeed experience it: "Though older "I" am
the same ‘person’ with the same ‘name’..." we say
and automatically tend to think…However only the name,
which is an empty designation or ‘label’ remains the same,
but nothing
of what that this label ‘names’ or signifies, whether mental
or physical remains the same
even
for a millisecond! All mental states and also
all
bodily material states
change momentarily!
Thus "we" are never exactly the same from one moment
to the next. Nor is anything external...!

Secondly:
A true
‘Self’ is assumed to be in full power over itself…
Otherwise: If this self have
no power over itself: Who or what has then power of this
self? If anything has power over the
self then this actually deserves more to be called a 'self'.
However realistically all events depend
on specific causes and not any imaginary ‘self’! Nothing is the same. Everything is changing.
Nothing is a static entity. Everything is a dynamic process.Nothing is
therefore under full control of any self. And nothing can
ever be
under full control of self

since everything is dependent on its own specific causes.

The "Ego-Me-I-Self" is a mere recursive auto-projection or
"self"-imagination of a non-real entity!

Thirdly:There is individuality:Only a single process of the
5 clusters of clinging(khandha)
conventionally called ‘a person’experiences the kammic after effects of it ‘own’ past activities…
Other clusters of clinging
conventionally called ‘other persons’
do not run into or experience these kammic effects.But there is no identity:Nothing within
or outside of this ever re-arising process of these 5 clusters of clinging
is ever
the ‘same’ or therefore ‘identical’, or
therefore an ‘identity’…
The ‘person’ is thus neither ever
the ‘same’,
nor ever ‘another’,
but just a selfless process of ever changing momentary states…Nothing of
such ephemeral transience is ever worth clinging to!

As the Buddha said (and only the Buddhas can re-discover
this):
"Sabbe Dhamma Anatta!"
=
All phenomena are no-self and core-less! All states
are Impersonal! There is no Agent, or Substance neither
behind,
nor within, nor without any Appearance...

'Blissful is solitude for one who is content, learned & who
see the True Dhamma.
Blissful is harmlessness towards all beings without
exception.
Blissful is freedom from any sensual urge whatsoever.
Yet, the supreme bliss, is the elimination of this abysmal
conceit “I am”!’ Udana
– Inspiration: II – 1